I sat with my dad for a while when he passed. Granted, he died at home, and so I was sitting on the side of the bed, talking to him. It was surreal, but I do not think it's harmful. It's hard to say that unilaterally, though, as everyone is a little different. For me, I've experienced a lot of death, but my father's was very much the hardest to take. And it's really a day by day thing for you now, I think. Some days, it will be easier. Some days will be harder.
Don't second-guess the choice for the DNR. I think you made the right choice, because she was not comfortable. Also, consider this: it is entirely possible that she would not have been able to be resuscitated anyhow. My mother-in-law died of cancer (in fact, it was pretty much every cancer BUT lung cancer, despite the fact that she smoked for the majority of her life), and there is no possible way that they could have kept her alive any longer than her own determination did. The reality is that, when a major illness is involved, there comes a point where the illness will in fact claim the person no matter how hard you try to stop it. So don't question your choice. You did the right thing.
As far as the remains go: they are what they are. They are remains. That which made her uniquely her, which animated her smile and put the life in her eyes, that part is still intact, safe, even if it's no longer within her body. Your memories of her ensure she lives on long past the hour and minute that she ceased to breathe. If those remains can help others, you know she'd be happy to help... and know that she's doing everything she can to try to fight the disease even though she's no longer animate. To me, that's a powerful thing, and it's something I deeply respect and appreciate.
Remember that she's always there to talk to. In fact, she's even more convenient now. She may not be able to talk back, but after a lifetime of being around her, you already can guess pretty well what she'd say. Just think about it for a while.
And most of all: get rest, eat right, do your normal daily things, and remember to live. She wouldn't want to see you suffering just because she's not physically standing there beside you. No mom does.